The Jamie Oliver-inspired drive towards healthy school meals is in danger of collapsing because changes to menus this autumn will lead to even more pupils deserting canteens for fast-food outlets, ministers are being warned.

Many schools will be forced to stop providing hot lunches because an exodus of older students will make the service no longer viable, claim local council catering firms, which provide the meals in thousands of schools across England.

From September secondary schools in England will only be allowed to sell food that meets high nutritional standards and pupils will have to buy a two-course lunch rather than just an individual item, such as a baked potato, as they do now.

The Local Authority Caterers Association (Laca) fears that both the healthy new meals, and the introduction of the "meal deal only" rule, will prove unpopular with sizeable numbers of pupils. Its members, firms who between them supply 2.5m school meals a day, are already having to contend with a drop in demand for lunches and sharp increases in the cost of many foods.

Sandra Russell, a former Laca president who is head of catering for Warwickshire county council, said there was a threat of a backlash from pupils. "Secondary school students are different to those from primaries. They are used to having a choice of things like a jacket potato bar or salad bar. So imposing a set meal may actually deter them from using the service," she said.

Some councils have seen a decline in the take-up of their school meals since the government ordered an overhaul to make them healthier following the agenda-setting Jamie's School Dinners series on Channel 4 in 2005.

Figures from the School Food Trust show that the proportion of secondary pupils eating a canteen lunch has fallen from 42.7% in 2005-06 to 37.6% in 2007-08, although primaries, which have already implemented the demanding nutritional standards, experienced a slight rise in the same period, from 42.3% to 43%.

Laca will hold a summit on Wednesday to highlight what its president, Neil Porter, says are providers' already shaky finances. Porter said Ed Balls, the children's secretary, had to act to avoid even greater numbers of secondary pupils eating unhealthy products for lunch. Laca wants only the main meal of the day to have to comply with the new standards from September and for schools to still be able to sell items such as pizza slices and sandwiches as single items.

But Judy Hargadon, the School Food Trust's chief executive, said the changes were "the final piece in the jigsaw to overhaul school meals". The nutritional standards were "tough but achievable", she said. "We have to succeed. There are no half-measures if we are going to dramatically improve the health, wellbeing and performance of children."